Received power level for tag processing prioritization

ABSTRACT

A method includes transmitting, by a radio frequency identification (RFID) reader, a series of RF signals to RFID tags in a time sequence. A first RF signal includes a first message for responding when received signal strength corresponds to a first power level and a second RF signal includes a second message for responding when the received signal strength of the RF signal corresponds to a second power level. The method further includes receiving, by the RFID reader, a first set of responses from a first set of RFID tags that received the first and second RF signals at a received signal strength corresponding to the first power level. The method further includes receiving, by the RFID reader, a second set of responses from a second set of RFID tags that received the first and second RF signals at a received signal strength corresponding to the second power level.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present U.S. Utility Patent Application claims priority pursuant to35 U.S.C. §120 as a continuation of U.S. Utility application Ser. No.14/644,471, entitled, “Method and Apparatus for Detecting RF FieldStrength”, filed on Mar. 11, 2015, which claims priority pursuant to 35U.S.C. §120 as a continuation of U.S. Utility application Ser. No.13/209,425, entitled “Method and Apparatus for Detecting RF FieldStrength”, filed Aug. 14, 2011, now U.S. Pat. No. 9,048,819, issued Jun.2, 2015, which is a Continuation-In-Part of application Ser. No.12/462,331, filed Aug. 1, 2009, now U.S. Pat. No. 8,081,043, issued Dec.20, 2011 (“Related Application”), which is in turn a Division ofapplication Ser. No. 11/601,085, filed Nov. 18, 2006, now U.S. Pat. No.7,586,385, issued Sep. 8, 2009 (“Related Patent”) (collectively,“Related References”). U.S. Utility application Ser. No. 13/209,425 alsoclaims priority pursuant to 35 U.S.C. §119(e) to U.S. ProvisionalApplication No. 61/428,170, filed Dec. 29, 2010, and U.S. ProvisionalApplication No. 61/485,732, filed May 13, 2011. The subject matter ofthe Related References and Related Applications, each in its entirety,is expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to detecting RF field strength,and, in particular, to detecting RF field strength in a passive RFIDsystem.

2. Description of the Related Art

In general, in the descriptions that follow, we will italicize the firstoccurrence of each special term of art that should be familiar to thoseskilled in the art of radio frequency (“RE”) communication systems. Inaddition, when we first introduce a term that we believe to be new orthat we will use in a context that we believe to be new, we will boldthe term and provide the definition that we intend to apply to thatterm. In addition, throughout this description, we will sometimes usethe terms assert and negate when referring to the rendering of a signal,signal flag, status bit, or similar apparatus into its logically true orlogically false state, respectively, and the term toggle to indicate thelogical inversion of a signal from one logical state to the other.Alternatively, we may refer to the mutually exclusive Boolean states aslogic_0 and logic_1. Of course, as is well known, consistent systemoperation can be obtained by reversing the logic sense of all suchsignals, such that signals described herein as logically true becomelogically false and vice versa. Furthermore, it is of no relevance insuch systems which specific voltage levels are selected to representeach of the logic states.

In accordance with our prior invention previously disclosed in theRelated References, the amplitude modulated (“AM”) signal broadcast bythe reader in an RFID system will be electromagnetically coupled to aconventional antenna, and a portion of the current induced in a tankcircuit is extracted by a regulator to provide operating power for allother circuits. Once sufficient stable power is available, the regulatorwill produce, e.g., a power-on-reset signal to initiate systemoperation. Thereafter, the method disclosed in the Related References,and the associated apparatus, dynamically varies the capacitance of avariable capacitor component of the tank circuit so as to dynamicallyshift the f_(R) of the tank circuit to better match the f_(C) of thereceived RF signal, thus obtaining maximum power transfer in the system.

In general, the invention disclosed in the Related References focusedprimarily on quantizing the voltage developed by the tank circuit as theprimary means of matching the f_(R) of the tank circuit to thetransmission frequency, f_(C), of the received signal. However, thisvoltage quantization is, at best, indirectly related to received signalfield strength. We submit that what is needed now is an effective andefficient method and apparatus for quantizing the received fieldstrength as a function of induced current. It is further desirable todevelop this field quantization in a form and manner that is suitablefor selectively varying the input impedance of the receiver circuit tomaximize received power, especially during normal system operation.Additionally, in light of the power sensitive nature of RFID systems, itis desirable to vary the input impedance with a minimum power loss.

BRIEF SUMMARY OF THE INVENTION

In accordance with the preferred embodiment of our invention, we providea sensing system for use in an RFID system. In general, the sensingsystem comprises an RFID tag and an RFID reader. In one embodiment, theRFID tag comprises a tank circuit having a selectively variableimpedance; and a tuning circuit adapted to dynamically vary theimpedance of the tank circuit, and to develop a first quantized valuerepresentative of the impedance of said tank circuit. In one alternateembodiment, the RFID tag comprises a detector circuit adapted to developa second quantized value as a function of a field strength of a receivedRF signal. In yet another embodiment, the RFID tag comprises both thetank and tuning circuit, and the detector circuit. In these embodiments,RFID reader is adapted selectively to retrieve one or both of the firstand second values, and, preferably, to use the retrieved values to sensechanges to an environment to which the RFID tag is exposed

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

My invention may be more fully understood by a description of certainpreferred embodiments in conjunction with the attached drawings inwhich:

FIG. 1 illustrates, in block diagram form, an RF receiver circuit havinga field strength detector constructed in accordance with an embodimentof our invention;

FIG. 2 illustrates, in block diagram form, a field strength detectorcircuit constructed in accordance with an embodiment of our invention;

FIG. 3 illustrates, in block schematic form, a more detailed embodimentof the field strength detector circuit shown in FIG. 2;

FIG. 4 illustrates, in flow diagram form, the sequencing of operationsin the field strength detector circuit shown in FIG. 3;

FIG. 5 illustrates, in graph form, the response of the field strengthdetector circuit shown in FIG. 3 to various conditions;

FIG. 6 illustrates, in block schematic form, an RF receiver circuitconstructed in accordance with another embodiment of our invention;

FIG. 7 illustrates, in flow diagram form, the sequencing of theoperations in the RF receiver circuit shown in FIG. 6;

FIG. 8 illustrates, in block schematic form, an alternativerepresentation of the impedance represented by the antenna and the tankcircuit of the exemplary RFID receiver circuit.

FIG. 9 illustrates, in block schematic form, an alternative exemplaryembodiment of the field strength detector circuit shown in FIG. 3.

FIG. 10 illustrates, in block schematic form, an alternative exemplaryembodiment of the field strength detector circuit shown in FIG. 3.

FIG. 11 illustrates, in block schematic form, an exemplary RFIDsub-system containing tag and reader.

In the drawings, similar elements will be similarly numbered wheneverpossible. However, this practice is simply for convenience of referenceand to avoid unnecessary proliferation of numbers, and is not intendedto imply or suggest that our invention requires identity in eitherfunction or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Shown in FIG. 1 is an RF receiver circuit 10 suitable for use in an RFIDapplication. As we have described in our Related References, an RFsignal electromagnetically coupled to an antenna 12 is received via atank circuit 14, the response frequency, f_(R), of which is dynamicallyvaried by a tuner 16 to better match the transmission frequency, f_(C),of the received RF signal, thus obtaining a maximum power transfer. Inparticular, as further noted in the Related Applications, the RMSvoltage induced across the tank circuit 14 by the received RF signal isquantized by tuner 16 and the developed quantization employed to controlthe impedance of the tank circuit 14. As also described in the RelatedReferences, the unregulated, AC current induced in the tank circuit 14by the received RF signal is conditioned by a regulator 18 to provideregulated DC operating power to the receiver circuit 10. In accordancewith our present invention, we now provide a field strength detector 20,also known as a power detector, adapted to develop a field-strengthvalue as a function of the field strength of the received RF signal. Aswe have indicated in FIG. 1, our field strength detector 20 is adaptedto cooperate with the regulator 18 in the development of thefield-strength value. As we shall disclose below, if desired, our fieldstrength detector 20 can be adapted to cooperate with the tuner 16 incontrolling the operating characteristics of the tank circuit 14.

Shown by way of example in FIG. 2 is one possible embodiment of ourfield strength or power detector 20. In this embodiment, we have chosento employ a shunt-type regulator 18 so that, during normal operation, wecan use the shunted ‘excess’ current as a reference against which wedevelop the field-strength value. In this regard, we use a reference 22first to develop a shunt current reference value proportional to theshunted current, and then to develop a mirrored current reference valueas a function of both the shunted current and a field strength referencecurrent provided by a digitally-controlled current source 24.Preferably, once the tuner 16 has completed its initial operatingsequence, whereby the f_(R) of the tank circuit 14 has beensubstantially matched to the f_(C) of the received signal, we thenenable a digital control 26 to initiate operation of the current source24 at a predetermined, digitally-established minimum field strengthreference current. After a predetermined period of time, control 26captures the mirrored current reference value provided by the currentreference 22, compares the captured signal against a predeterminedthreshold value, and, if the comparison indicates that the fieldstrength reference current is insufficient, increases, in accordancewith a predetermined sequence of digital-controlled increments, thefield strength reference current; upon the comparison indicating thatthe field strength reference current is sufficient, control 26 will, atleast temporarily, cease operation.

In accordance with our invention, the digital field-strength valuedeveloped by control 26 to control the field strength current source 24is a function of the current induced in the tank circuit 14 by thereceived RF signal. Once developed, this digital field-strength valuecan be employed in various ways. For example, it can be selectivelytransmitted by the RFID device (using conventional means) back to thereader (not shown) for reference purposes. Such a transaction can beeither on-demand or periodic depending on system requirements. Imaginefor a moment an application wherein a plurality of RFID tag devices isdistributed, perhaps randomly, throughout a restricted, 3-dimensionalspace, e.g., a loaded pallet. Imagine also that the reader is programmedto query, at an initial field strength, all tags “in bulk” and tocommand all tags that have developed a field-strength value greater thana respective field-strength value to remain ‘silent’. By performing asequence of such operations, each at an increasing field strength, thereader will, ultimately, be able to isolate and distinguish those tagsmost deeply embedded within the space; once these ‘core’ tags have beenread, a reverse sequence can be performed to isolate and distinguish alltags within respective, concentric ‘shells’ comprising the space ofinterest. Although, in all likelihood, these shells will not be regularin either shape or relative volume, the analogy should still be apt.

In FIG. 3, we have illustrated one possible embodiment of our fieldstrength detector 20 a. In general, we have chosen to use a shuntcircuit 18 a to develop a substantially constant operating voltage levelacross supply node 28 and ground node 30. Shunt regulators of this typeare well known in the art, and typically use Zener diodes, avalanchebreakdown diodes, diode-connected MOS devices, and the like.

As can be seen, we have chosen to implement current reference 22 in theform of a current mirror circuit 22 a, connected in series with shuntcircuit 18 a between nodes 28 and 30. As is typical, current mirrorcircuit 22 a comprises a diode-connected reference transistor 32 and amirror transistor 34. If desired, a more sophisticated circuit such as aWidlar current source may be used rather than this basic two-transistorconfiguration. For convenience of reference, we have designated thecurrent shunted by shunt circuit 18 a via reference transistor 32 asi_(R); similarly, we have designated the current flowing through mirrortransistor 34 as i_(R)/N, wherein, as is known, N is the ratio of thewidths of reference transistor 32 and mirror transistor 34.

We have chosen to implement the field strength current source 24 as aset of n individual current sources 24 a, each connected in parallelbetween the supply node 28 and the mirror transistor 34. In general,field strength current source 24 a is adapted to source current at alevel corresponding to an n-bit digital control value developed by acounter 38. In the illustrated embodiment, wherein n=5, field strengthcurrent source 24 a is potentially capable of sourcing thirty-twodistinct reference current levels. We propose that the initial, minimumreference current level be selected so as to be less than the currentcarrying capacity of the mirror transistor 34 when the shunt circuit 18a first begins to shunt excess induced current through referencetransistor 32; that the maximum reference current level be selected soas to be greater than the current carrying capacity of the mirrortransistor 34 when the shunt circuit 18 a is shunting a maximumanticipated amount of excess induced current; and that the intermediatereference current levels be distributed relatively evenly between theminimum and maximum levels. Of course, alternate schemes may bepracticable, and, perhaps, desirable depending on system requirements.

Within control 26 a, a conventional analog-to-digital converter (“ADC”)40, having its input connected to a sensing node 36, provides a digitaloutput indicative of the field strength reference voltage, v_(R),developed on sensing node 36. In one embodiment, ADC 40 may comprise acomparator circuit adapted to switch from a logic_0 state to a logic_1when sufficient current is sourced by field strength current source 24 ato raise the voltage on sensing node 36 above a predetermined referencevoltage threshold, v_(th). Alternatively, ADC 40 may be implemented as amulti-bit ADC capable of providing higher precision regarding thespecific voltage developed on sensing node 36, depending on therequirements of the system. Sufficient current may be characterized asthat current sourced by the field strength current source 24 a or sunkby mirror transistor 34 such that the voltage on sensing node 36 isaltered substantially above or below a predetermined reference voltagethreshold, v_(th). In the exemplary case of a simple CMOS inverter,v_(th) is, in its simplest form, one-half of the supply voltage (VDD/2).Those skilled in the art will appreciate that v_(th) may byappropriately modified by altering the widths and lengths of the devicesof which the inverter is comprised. In the exemplary case a multi-bitADC, v_(th) may be established by design depending on the systemrequirements and furthermore, may be programmable by the system.

In the illustrated embodiment, a latch 42 captures the output state ofADC 40 in response to control signals provided by a clock/controlcircuit 44. If the captured state is logic_0, the clock/control circuit44 will change counter 38 to change the reference current being sourcedby field strength current source 24 a; otherwise clock/control circuit44 will, at least temporarily, cease operation. However,notwithstanding, the digital field-strength value developed by counter38 is available for any appropriate use, as discussed above.

By way of example, we have illustrated in FIG. 4 one possible generaloperational flow of our field strength detector 20 a. Upon activation,counter 38 is set to its initial digital field-strength value (step 48),thereby enabling field strength current source 24 a to initiatereference current sourcing at the selected level. After an appropriatesettling time, the field strength reference voltage, v_(R), developed onsensing node 36 and digitized by ADC 40 is captured in latch 42 (step50). If the captured field strength reference voltage, v_(R), is lessthan (or equal to) the predetermined reference threshold voltage,v_(th), clock/control 44 will change counter 38 (step 54). This processwill repeat, changing the reference current sourced by field strengthcurrent source 24 a until the captured field strength reference voltage,v_(R), is greater than the predetermined reference threshold voltage,v_(th), (at step 52), at which time the process will stop (step 56). Asillustrated, this sweep process can be selectively reactivated asrequired, beginning each time at either the initial field-strength valueor some other selected value within the possible range of values asdesired.

The graph illustrated in FIG. 5 depicts several plots of the voltagedeveloped on sensing node 36 as the field strength detector circuit 20 asweeps the value of counter 38 according to the flow illustrated in FIG.4. As an example, note that the curve labeled “A” in FIG. 5 begins at alogic_0 value when the value of counter 38 is at a minimum value such as“1” as an exemplary value. Subsequent loops though the sweep loopgradually increase the field strength reference voltage on sensing node36 until counter 38 reaches a value of “4” as an example. At this point,the “A” plot in FIG. 5 switches from a logic_0 value to a logic_1 value,indicating that the field strength reference voltage, v_(R), on sensingnode 36 has exceeded the predetermined reference threshold voltage,v_(th). Other curves labeled “B” through “D” depict incrementalincreases of reference currents, i_(R), flowing through reference device32, resulting in correspondingly higher mirrored currents flowingthrough mirror device 34. This incrementally higher mirror currentrequires field strength current source 24 to source a higher currentlevel which in turn corresponds to higher values in counter 38. Thus, itis clear that our invention is adapted to effectively and efficientlydevelop a digital representation of the current flowing through sensingnode 36 that is suitable for any appropriate use.

One such use, as discussed earlier, of our field strength detector 20 isto cooperate with tuner 16 in controlling the operating characteristicsof the tank circuit 14. FIG. 6 illustrates one possible embodiment wherereceiver circuit 10 a uses a field strength detector 20 b speciallyadapted to share with tuner 16 a the control of the tank circuit 14. Inour Related References we have disclosed methods, and related apparatus,for dynamically tuning, via tuner 16 a, the tank circuit 14 so as todynamically shift the f_(R) of the tank circuit 14 to better match thef_(C) of the received RF signal at antenna 12. By way of example, wehave shown in FIG. 6 how the embodiment shown in FIG. 3 of our RelatedPatent may be easily modified by adding to tuner 16 a a multiplexer 58to facilitate shared access to the tuner control apparatus. Shown inFIG. 7 is the operational flow (similar to that illustrated in FIG. 4 inour Related Patent) of our new field strength detector 20 b uponassuming control of tank circuit 14.

In context of this particular use, once tuner 16 a has completed itsinitial operating sequences as fully described in our Related Patent,and our field strength detector 20 b has performed an initial sweep (asdescribed above and illustrated in FIG. 4) and saved in a differentiator60 a base-line field-strength value developed in counter 38,clock/control 44 commands multiplexer 58 to transfer control of the tankcircuit 16 a to field strength detector 20 b (all comprising step 62 inFIG. 7). Upon completing a second current sweep, differentiator 60 willsave the then-current field-strength value developed in the counter 38(step 64). Thereafter, differentiator 60 will determine the polarity ofthe change of the previously saved field-strength value with respect tothe then-current field-strength value developed in counter 38 (step 66).If the polarity is negative (step 68), indicating that the currentfield-strength value is lower than the previously-saved field-strengthvalue, differentiator 60 will assert a change direction signal;otherwise, differentiator 60 will negate the change direction signal(step 70). In response, the shared components in tuner 16 a downstreamof the multiplexer 58 will change the tuning characteristics of tankcircuit 14 (step 72) (as fully described in our Related References).Now, looping back (to step 64), the resulting change of field strength,as quantized is the digital field-strength value developed in counter 38during the next sweep (step 64), will be detected and, if higher, willresult in a further shift in the f_(R) of the tank circuit 14 in theselected direction or, if lower, will result in a change of direction(step 70). Accordingly, over a number of such ‘seek’ cycles, ourinvention will selectively allow the receiver 10 a to maximize receivedfield strength even if, as a result of unusual factors, the f_(R) of thetank circuit 14 may not be precisely matched to the f_(C) of thereceived RF signal, i.e., the reactance of the antenna is closelymatched with the reactance of the tank circuit, thus achieving maximumpower transfer. In an alternative embodiment, it would be unnecessaryfor tuner 16 a to perform an initial operating sequence as fullydescribed in our Related Patent. Rather, field strength detector 20 bmay be used exclusively to perform both the initial tuning of thereceiver circuit 10 a as well as the subsequent field strengthdetection. Note that the source impedance of antenna 12 and loadimpedance of tank circuit 14 may be represented alternatively inschematic form as in FIG. 8, wherein antenna 12 is represented asequivalent source resistance R_(S) 74 and equivalent source reactanceX_(S) 76, and tank circuit 14 is represented as equivalent loadresistance R_(L) 78 and equivalent, variable load reactance X_(L) 80.

In FIG. 9, we have illustrated an alternate embodiments of our fieldstrength detector illustrated in FIG. 3. Here, as before, shunt circuit18 b is used to develop a substantially constant operating voltage levelacross supply node 28 and ground node 30. Also, as before, the currentreference 22 is implemented as a current mirror circuit 22 b connectedin series with shunt circuit 18 b between nodes 28 and 30. However, inthis embodiment, the field strength current source comprises a resistivecomponent 84 adapted to function as a static resistive pull-up device.Many possible implementations exist besides a basic resistor, such as along channel length transistor, and those skilled in the art willappreciate the various implementations that are available to accomplishanalogous functionality. The field strength voltage reference v_(R)developed on sensing node 36 will be drawn to a state near the supplyvoltage when the mirrored current flowing though transistor 34 isrelatively small, e.g. close to zero amps, indicating a weak fieldstrength. As the field strength increases, the current flowing throughmirror transistor 34 will increase, and the field strength voltagereference v_(R), developed on sensing node 36 will drop proportionallyto the mirrored current flowing through mirror transistor 34 as i_(R)/N.ADC 40, having its input connected to sensing node 36, provides adigital output indicative of the field strength reference voltage,v_(R), developed on sensing node 36, as described previously.

In this alternate embodiment, latch 42 captures the output state of ADC40 in response to control signals provided by a clock/control circuit44. As disclosed earlier, the ADC 40 may comprise a comparator circuit.In this instance, ADC 40 is adapted to switch from a logic_1 state to alogic_0 when sufficient current is sunk by mirror transistor 34 to lowerthe voltage on sensing node 36 below a predetermined reference voltagethreshold, v_(th). Alternatively, ADC 40 may be implemented as amulti-bit ADC capable of providing higher precision regarding thespecific voltage developed on sensing node 36, depending on therequirements of the system.

Comparator 82 subsequently compares the captured output state held inlatch 42 with a value held in counter 38 that is selectively controlledby clock/control circuit 44. In response to the output generated bycomparator 82, clock/control circuit 44 may selectively change the valueheld in counter 38 to be one of a higher value or a lower value,depending on the algorithm employed. Depending upon the implementationof counter 38 and comparator 82, clock/control circuit 44 may alsoselectively reset the value of counter 38 or comparator 82 or both. Thedigital field-strength value developed by counter 38 is available forany appropriate use, as discussed above.

In FIG. 10, we have illustrated another alternate embodiment of ourfield strength detector illustrated in FIG. 3. Here, as before, shuntcircuit 18 c is used to develop a substantially constant operatingvoltage level across supply node 28 and ground node 30. In thisembodiment, the current reference 22 is implemented as a resistivecomponent 86 that functions as a static pull-down device. Many possibleimplementations exist besides a basic resistor, such as a long channellength transistor and those skilled in the art will appreciate thevarious implementations that are available to accomplish analogousfunctionality. The field strength voltage reference vR developed onsensing node 36 will be drawn to a state near the ground node when thecurrent flowing though shunt circuit 18 c is relatively small, e.g.close to zero amps, indicating a weak field strength. As the fieldstrength increase, the current flowing through shunt circuit 18 c willincrease, and the field strength voltage reference v_(R) developed onsensing node 36 will rise proportionally to the current flowing throughshunt circuit 18 c. ADC 40, having its input connected to a sensing node36, provides a digital output indicative of the field strength referencevoltage, v_(R), developed on sensing node 36, as described previously.

In this alternate embodiment, latch 42 captures the output state of ADC40 in response to control signals provided by a clock/control circuit44. As disclosed earlier, the ADC 40 may comprise a comparator circuit.In this instance, ADC 40 is adapted to switch from a logic_0 state to alogic_1 when sufficient current is sourced by shunt circuit 18 c toraise the voltage on sensing node 36 above a predetermined referencevoltage threshold, v_(th). Alternatively, ADC 40 may be implemented as amulti-bit ADC capable of providing higher precision regarding thespecific voltage developed on sensing node 36, depending on therequirements of the system.

Comparator 82 subsequently compares the captured output state held inlatch 42 with a value held in counter 38 that is selectively controlledby clock/control circuit 44. In response to the output generated bycomparator 82, clock/control circuit 44 may selectively change the valueheld in counter 38 to be one of a higher value or a lower value,depending on the algorithm employed. Depending upon the implementationof counter 38 and comparator 82, clock/control circuit 44 may alsoselectively reset the value of counter 38 or comparator 82 or both. Thedigital field-strength value developed by counter 38 is available forany appropriate use, as discussed above.

In another embodiment, our invention may be adapted to sense theenvironment to which a tag is exposed, as well as sensing changes tothat same environment. As disclosed in our Related References, theauto-tuning capability of tuner 16 acting in conjunction with tankcircuit 14 detects antenna impedance changes. These impedance changesmay be a function of environmental factors such as proximity tointerfering substances, e.g., metals or liquids, as well as a functionof a reader or receiver antenna orientation. Likewise, as disclosedherein, our field strength (i.e., received power) detector 20 may beused to detect changes in received power (i.e., field strength) as afunction of, for example, power emitted by the reader, distance betweentag and reader, physical characteristics of materials or elements in theimmediate vicinity of the tag and reader, or the like. Sensing theenvironment or, at least, changes to the environment is accomplishedusing one or both of these capabilities.

As an example, the tag 88 of FIG. 11, contains both a source tag antenna12 (not shown, but see, e.g., FIG. 6) and a corresponding load chip tankcircuit 14 (not shown, but see, e.g., FIG. 6). Each contains bothresistive and reactive elements as discussed previously (see, e.g., FIG.8). A tag 88 containing such a tank circuit 14 mounted on a metallicsurface will exhibit antenna impedance that is dramatically differentthan the same tag 88 in free space or mounted on a container of liquid.Table 1 displays exemplary values for impedance variations in bothantenna source resistance 74 as well as antenna source reactance 76 as afunction of frequency as well as environmental effects at an exemplaryfrequency:

TABLE 1 Antenna Impedance Variations In Free Air 860 MHz 910 MHz 960 MHzR_(s) 1.9 2.5 3.7 X_(s) 124 136 149 @ 910 MHz Free Air On Water On MetalR_(s) 2.5 26 1.9 X_(s) 136 136 27

The tuner circuit 16 of our invention as disclosed in the RelatedReferences automatically adjusts the load impendence by adjusting loadreactance 80 (see, e.g., FIG. 8) to match source antenna impedancerepresented by source resistance 74 (see, e.g., FIG. 8) and sourcereactance 76 (see, e.g., FIG. 8). As previously disclosed, matching ofthe chip load impedance and antenna source impedance can be performedautomatically in order to achieve maximum power transfer between theantenna and the chip. My invention as disclosed in the RelatedReferences contained a digital shift register 90 for selectivelychanging the value of the load reactive component, in the present case avariable capacitor, until power transfer is maximized. This digitalvalue of the matched impendence may be used either internally by the tag88, or read and used by the reader 92, to discern relative environmentalinformation to which the tag 88 is exposed. For example, tag 88 maycontain a calibrated look-up-table within the clock/control circuit 44which may be accessed to determine the relevant environmentalinformation. Likewise, a RFID reader 92 may issue commands (seetransaction 1 in FIG. 11) to retrieve (see transaction 2 in FIG. 11) thevalues contained in digital shift register 90 via conventional means,and use that retrieved information to evaluate the environment to whichtag 88 is exposed. The evaluation could be as simple as referencingfixed data in memory that has already been stored and calibrated, or ascomplex as a software application running on the reader or its connectedsystems for performing interpretive evaluations.

Likewise, consider a tag 88 containing our field strength (i.e.,received power) detector 20 (not shown, but, e.g., see FIG. 6) whereinthe method of operation of the system containing the tag 88 calls forour field strength detector 20 to selectively perform its sweep functionand developing the quantized digital representation of the current viathe method discussed earlier. As illustrated in FIG. 11, counter 38 willcontain the digital representation developed by our field strengthdetector 20 of the RF signal induced current, and may be used eitherinternally by the tag 88, or read and used by the reader 92, to discernrelative environmental information to which the tag 88 is exposed. Forexample, reader 92 may issue a command to the tag 88 (see transaction 1in FIG. 11) to activate tuner 16 and/or detector 20 and, subsequent tothe respective operations of tuner 16 and/or detector 20, receive (seetransaction 2 in FIG. 11) the digital representations of either thematched impedance or the maximum current developed during thoseoperations. Once again, this digital value of the field strength storedin the counter 38 may be used either internally by the tag 88, or readand used by the reader 92, to discern relative environmental informationto which the tag 88 is exposed. For example, tag 88 may contain acalibrated look-up-table within the clock and control block 44 which maybe accessed to determine the relevant environmental information.Likewise, a RFID reader may issue commands to retrieve the valuescontained in digital shift register 90, and use that retrievedinformation to evaluate the environment to which tag 88 is exposed. Theevaluation could be as simple as referencing fixed data in memory thathas already been stored and calibrated, or as complex as a softwareapplication running on the reader or its connected systems forperforming interpretive evaluations. Thus, the combining of thetechnologies enables a user to sense the environment to which a tag 88is exposed as well as sense changes to that same environment.

Thus, it is apparent that we have provided an effective and efficientmethod and apparatus for quantizing the received RF field strength as afunction of induced current. We have developed this field quantizationin a form and manner that is suitable for selectively varying theimpedance of the tank circuit to maximize received power, especiallyduring normal system operation. Those skilled in the art will recognizethat modifications and variations can be made without departing from thespirit of our invention. Therefore, we intend that our inventionencompass all such variations and modifications as fall within the scopeof the appended claims.

What we claim is:
 1. A method comprises: transmitting, by a radiofrequency identification (RFID) reader, a series of RF signals to aplurality of RFID tags in a time sequence, wherein a first RF signal ofthe series of RF signals includes a first message that includes a firstresponse request when received signal strength of the RF signalcorresponds to a first power level and wherein a second RF signal of theseries of RF signals includes a second message that includes a secondresponse request when the received signal strength of the RF signalcorresponds to a second power level; receiving, by the RFID reader, afirst set of responses from a first set of RFID tags, wherein each RFIDtag of the first set of RFID tags received the first and second RFsignals at a received signal strength corresponding to the first powerlevel; and receiving, by the RFID reader, a second set of responses froma second set of RFID tags, wherein each RFID tag of the second set ofRFID tags received the first and second RF signals at a received signalstrength corresponding to the second power level.
 2. The method of claim1 further comprises: receiving, by a first RFID tag of the plurality ofRFID tags, the series of RF signals; determining, by the first RFID tag,power levels for each RF signal of the series of RF signals; and whenthe power level of one of the RF signals corresponds to the first powerlevel: transmitting, by the first RFID tag, a response to the first RFsignal; and remaining, by the first RFID tag, silent in response to thesecond RF signal.
 3. The method of claim 2 further comprises: when thepower level of the one of the RF signals corresponds to the second powerlevel: transmitting, by the first RFID tag, a second response to thesecond RF signal; and remaining, by the first RFID tag, silent inresponse to the first RF signal.
 4. The method of claim 1 furthercomprises: receiving, by the RFID reader, a third set of responses froma third set of RFID tags, wherein each RFID tag of the third set of RFIDtags received the first, the second, and a third RF signal at a receivedsignal strength corresponding to the third power level, wherein thethird RF signal is from the series of RF signals and includes a thirdmessage that includes a third response request when the received signalstrength of the RF signal corresponds to the third power level.
 5. Themethod of claim 1 further comprises: determining, by the RFID reader,that RFID tags of the second set of RFID tags are physically closer tothe RFID reader than RFID tags of the first set of RFID tags when thefirst power level is less than the second power level.
 6. The method ofclaim 1 further comprises: adjusting, by the RFID reader, a degree ofcorrespondence of the received signal strength of the RF signalcorresponding to the first power level such that the first set of RFIDtags includes a single RFID tag.
 7. The method of claim 1 furthercomprises: receiving a response from the first or the second set ofresponses, wherein the response includes one or more of: a receivedsignal strength indication, an indication of an environmental condition,and an identifier of an RFID tag.
 8. A radio frequency identification(RFID) reader comprises: a transmitter operable to transmit a series ofRF signals to a plurality of RFID tags in a time sequence, wherein afirst RF signal of the series of RF signals includes a first messagethat includes a first response request when received signal strength ofthe RF signal corresponds to a first power level and wherein a second RFsignal of the series of RF signals includes a second message thatincludes a second response request when the received signal strength ofthe RF signal corresponds to a second power level; a receiver operableto: receive a first set of responses from a first set of RFID tags,wherein each RFID tag of the first set of RFID tags received the firstand second RF signals at a received signal strength corresponding to thefirst power level; and receive a second set of responses from a secondset of RFID tags, wherein each RFID tag of the second set of RFID tagsreceived the first and second RF signals at a received signal strengthcorresponding to the second power level; and a control circuit operableto: generate the first and second messages; and process the first andsecond set of responses.
 9. The RFID reader of claim 8 furthercomprises: the receiver is further operable to: receive a third set ofresponses from a third set of RFID tags, wherein each RFID tag of thethird set of RFID tags received the first, the second, and a third RFsignal at a received signal strength corresponding to the third powerlevel, wherein the third RF signal is from the series of RF signals andincludes a third message that includes a third response request when thereceived signal strength of the RF signal corresponds to the third powerlevel; and the control circuit is further operable to: generate thethird message; and process the third set of responses.
 10. The RFIDreader of claim 8, wherein the control circuit is further operable to:determine that RFID tags of the second set of RFID tags are physicallycloser to the RFID reader than RFID tags of the first set of RFID tagswhen the first power level is less than the second power level.
 11. TheRFID reader of claim 8, wherein the control circuit is further operableto: adjust a degree of correspondence of the received signal strength ofthe RF signal corresponding to the first power level such that the firstset of RFID tags includes a single RFID tag.
 12. The RFID reader ofclaim 8, wherein the control circuit is further operable to: interpret aresponse from the first or the second set of responses as one or moreof: a received signal strength indication, an indication of anenvironmental condition, and an identifier of an RFID tag.
 13. A radiofrequency identification (RFID) tag comprises: a receiver operable toreceive a series of RF signals in a time sequence, wherein a first RFsignal of the series of RF signals includes a first message thatincludes a first response request when received signal strength of theRF signal corresponds to a first power level and wherein a second RFsignal of the series of RF signals includes a second message thatincludes a second response request when the received signal strength ofthe RF signal corresponds to a second power level; a power leveldetection circuit operable to determine power levels for each RF signalof the series of RF signals; a control circuit operable to: when thepower level of one of the RF signals corresponds to the first powerlevel: generate a response to the first RF signal; and remain silent inresponse to the second RF signal; and a transmitter operable to transmita response RF signal that includes the response.
 14. The RFID tag ofclaim 13 further comprises: the control circuit is further operable to:when the power level of the one of the RF signals corresponds to thesecond power level: generate a second response to the second RF signal;and remain silent in response to the first RF signal; and thetransmitter is further operable to transmit a second response RF signalthat includes the second response.
 15. The RFID tag of claim 13, whereinthe control circuit is further operable to: generate the response torepresent one of: a received signal strength indication, an indicationof an environmental condition, and an identifier of an RFID tag.